1. Field of the Invention
The present invention relates to a membrane electrode assembly (MEA), a manufacturing method thereof and a polymer electrolyte fuel cell (PEFC) using the same.
2. Description of the Related Art
A fuel cell is a power generation system in which a reverse reaction of electrolysis of water was performed with a fuel gas (including hydrogen) and an oxidant gas (including oxygen) at an electrode having a catalyst to produce heat and electric power. Fuel cells are attracting attention as a clean energy source of the future and have specific features relative to conventional systems such as high efficiency, a small impact on the environment and a small noise. According to the types of ion conductor, fuel cells are divided into several types including PEFC, in which an ion conducting polymer membrane is used.
Among various fuel cells, PEFC is considered as a promising fuel cell for use in a vehicle and a household stationary power supply etc. and is being developed widely in recent years. A complex unit which has a pair of polymer electrode layers on both sides of a polymer electrolyte and which is called a membrane electrode assembly (MEA) is arranged between a pair of separators, on which gas flow paths for supplying a fuel gas including hydrogen to one of the electrodes and an oxidant gas including oxygen to the other electrode is formed, in the PEFC. The electrode for supplying a fuel gas is called a fuel electrode, whereas the electrode for supplying an oxidant gas is called an air electrode hereafter. Each of the electrodes includes an electrode catalyst layer, which has stacked electrolytes along with carbon particles on which a catalyst such as a noble metal of platinum group is loaded, and a gas diffusion layer which has gas permeability and electron conductivity.
In order to improve performances of such a PEFC, a variety of manufacturing methods of an MEA are conventionally investigated. For example, the following methods are known: a method in which an electrode with a catalyst layer is formed by coating a coating liquid on an ion-exchange membrane followed by fabricating an MEA by combining the electrode and an ion-exchange membrane by a heat treatment such as hot-pressing, a method in which a catalyst layer is formed on a substrate film which is arranged besides an ion-exchange membrane and then the catalyst layer is transferred to the ion-exchange membrane by stacking an ion-exchange membrane onto the catalyst layer and performing hot-pressing, a method in which an electrode sheet having a catalytic function and gas diffusion properties is fabricated and then the sheet and an ion-exchange membrane are combined together, and a method in which a pair of half cells (a half cell is a pre-product obtained by forming a catalyst layer on an ion-exchange membrane) are fabricated and they are press-laminated together with both ion-exchange membrane's sides facing each other so that an MEA is obtained.
These manufacturing methods of an MEA, however, have a problem of decrease in production efficiency due to a long tact time since the ion-exchange membrane and the electrode catalyst layer are combined by thermocompression such as hot press, which may often be a critical (bottleneck) process. The tact time is shortened by performing the thermocompression with a higher temperature and higher pressure. In such a case, however, the catalyst layer may become too dense to retain diffusion properties and drainage properties, and the polymer electrolyte membrane may be degraded by heat so that the ion conductivity and mechanical strength is decreased. In the method in which an electrode catalyst layer is formed by coating a coating liquid on the gas diffusion layer, which may be used most among the methods mentioned above, there are severe risks of gas leakage, MEA voltage falling in a closed circuit, and shorting the anode and cathode. This is because the gas diffusion layer is normally a porous carbon paper or carbon felt, and some carbon fibers projecting to the surface of the gas diffusion layer may damage the electrode catalyst layer or even the ion-exchange membrane. Moreover, since a surface roughness of a carbon paper or carbon felt is generally larger than a thickness of the catalyst layer, it is difficult by this method to manufacture an MEA using a thin ion-exchange membrane which has a thickness, for example, less than (or equal to) 20 μm.
In contrast, a manufacturing method of a sequential-stacking type MEA, in which a first electrode catalyst layer is formed followed by forming a polymer electrolyte layer next and a second electrode catalyst layer last, has a short tact time, high production efficiency, and a low manufacturing cost. This manufacturing method, however, has problems of a decrease in a mechanical strength of the MEA, and inferior gas diffusion properties and drainage properties of the electrode catalyst layer. The gas diffusion properties and drainage properties get worse particularly when a high output power operation is performed.    <Patent document 1> JP-A-2004-047489    <Patent document 2> JP-A-2005-294123